Polymeric materials degrade with age, especially in severe environmental conditions. Degradation may be (1) chemical, such as chain scission and cross-linking, (2) physical, such as loss of volatile fractions and environmentally induced macroscopic changes such as crystallinity, and physical changes due to a tendency for thermodynamic equilibrium, and (3) mechanical damage such as cracking and delamination. Due to the increasing use of polymers in consumer products, transportation industries, military applications and commercial and industrial processes, there is a growing need for monitoring of the condition and remaining age of these materials before unsafe conditions or degraded performance occurs.
Composite materials find use where mechanical properties such as strength and rigidity are important. Non-destructive health monitoring of composite materials includes visual inspections, radiography, and acoustic methods. While these methods are effective for some applications, they suffer several serious disadvantages. For example, visual inspections are useful only on components with full visual access, and are not effective for flaws below the surface of the component. Radiography requires access to the components and it is time consuming and requires special training for evaluation of the results. The equipment is very expensive and subject to considerable controls to prevent radiation damage to personnel and equipment. Acoustic methods such as ultrasonic inspections require specialized equipment, considerable operator training, and good access to the component. All of these methods are effective only in detecting actual mechanical flaws such as cracks, and are not effective in predicting deterioration of the composite prior to development of the mechanical flaws.
An improved method is needed to monitor the health of composite materials which overcomes the drawbacks of current methods.